A fat or oil containing diacylglycerol at a high concentration is known to have physiological benefits, such as a very low degree of accumulation in the human body (JP-A-10-176181), and therefore is widely used as an edible oil. It has already been known that diacylglycerol is produced by chemical or enzymatic esterification using a fatty acid and glycerine as the starting materials (JP-A-1-71495) or by chemical or enzymatic glycerolysis using a fat or oil and glycerine as the starting materials (WO 03/29392 and JP-A-63-133992).
Some of the diacylglycerol-rich fats or oils produced by such methods, however, contain impurities, such as fatty acid, monoacylglycerol and odor components. For the diacylglycerol-rich fat or oil to be used as an edible oil, it is desirable to have a better flavor by reducing such impurities.
Among the methods of producing a diacylglycerol-rich fat or oil, there is a technique by which the fat or oil after the completion of the reaction is further purified and the monoacylglycerol recovered is added to the starting material of the subsequent cycle of the reaction (JP-A-8-294394). This technique is aimed to improve the speed of the esterification by increasing solubility of the glycerine in the fatty acid phase by adding the monoacylglycerol which has been collected upon removal of the impurities unsuitable for an edible oil by distillation under high vacuum conditions to the reaction system of the subsequent reaction cycle. Besides, there is another method by which the glycerine and monoacylglycerol remaining in the reaction system after completion of chemical glycerolysis using a fat or oil and glycerine as the starting materials are recovered by steam distillation or molecular distillation for use in the subsequent cycle of the reaction (WO 03/29392). This method is economically advantageous in that this method does not require the step of decomposing the fat or oil for use of the decomposition products in the subsequent esterification. Further, there is a technique requiring that an esterification reaction be performed after the partial hydrolysis of fat or oil without distillation, so that a trace of substances, such as phytosterol, could still be contained in the diacylglycerol product (JP-A-11-123097). While this technique has such an advantage, it should be noted that the concentration of the diacylglycerol obtained thereby is too low, and no consideration is made for re-use of the unreacted substances or the by-products.